FIG. 1 is a block diagram of a prior art super-heterodyne receiver with single analogue conversion. The receiver comprises an antenna band pass filter 101 arranged to filter an incoming analogue radio frequency (RF) signal, a low noise amplifier 102 arranged to amplify the incoming analogue RF signal, a band pass filter 103 acting as an intermediate frequency (IF) image reject filter to filter the amplified analogue RF signal, a frequency conversion block or mixer 104, which by aid of an oscillator 105 that provides the local oscillator frequency, is arranged to down convert the filtered analogue RF signal to an analogue IF signal, a band pass filter block comprising two band pass filter 106, 108 and an amplifier 107 and arranged to band pass filter and amplify the analogue IF signal, an analogue-to-digital (AD) converter 109 driven by a sampling clock 110 and arranged to convert the analogue IF signal to a digital signal 111, and a digital processing block 112 arranged to convert the digital signal to I- and Q-components to access a message in the incoming RF signal.
The receiver of FIG. 1 for a multicarrier receiver application is basically a double conversion receiver where the frequency conversion block 104 converts the analogue RF signal to an analogue IF signal, which is received by the AD converter 109 in one of the AD converter Nyquist zones. The second conversion is made in a digital domain utilizing e.g. a numerical controlled oscillator (NCO) multiplier with complex numbers to convert the output signal from the AD converter 109 to IF=0 to recover the I/Q modulation message in the signal.
The advantage of the super-heterodyne receiver is that no I/Q errors in the down-conversion process is introduced and the I/Q signal is obtained after a numerically controlled down conversion in the digital domain in contrary to the direct down conversion or homodyne principle were analogue modulators will introduce inaccuracies that can make homodyne receivers difficult to apply to multi carrier or multi signal receivers.
FIGS. 2a-b are Nyquist zone diagrams related to the operation of the prior art super-heterodyne receiver utilizing an AD converter 109 with sampling frequency fs. FIG. 2a shows the input signal scenario to the AD converter 109. A part of the second Nyquist zone 201 is selected to be used for the incoming analogue IF signal 203. The Nyquist zone 202 is the mirror zone for the real AD converter output signal digital since the response from the AD converter 109 in the frequency domain is mirrored in the Nyquist frequency fs/2. Signals 204, 205, and 206 are signals in adjacent Nyquist zones of the AD converter 109. FIG. 2b shows the resulting AD converter output scenario. The real AD converter output signal will receive the signal 203 and its real mirror image 209 if the disturbance signals 204, 205, and 206 are filtered away so they do not provide any real signals with mirror images in the AD converter output spectrum. Such disturbing or undesired signals are indicates by reference numerals 207 and 208.
Those parts of the Nyquist zone 201 which contain signals from other Nyquist zones of the AD converter 109 can not be used if not the band pass filtering before the AD converter 109 effectively blocks such signals. Therefore, in order to have as much receiver band width as possible in a Nyquist zone, very high Q band pass filtering has to be applied. The required slopes for such band pass filtering are indicated by 210 in FIG. 2a. Due to that filter problem the usage of a Nyquist zone bandwidth is normally limited to 50-70%.